I vote for SS mesh to block the overflow. Sponges clog often and quickly which would result in an overflow problem as the return pump is quickly outrunning the overflow. SS mesh last long between cleanings. Since you aren't keeping micro fish or shrimp you can even opt for as mesh with bigger gaps that would require less cleaning.
I agree. Foam on an overflow will slow down flow over time and may cause the thing to clog at some point (likely when your on vacation), so will screen with fine holes, but a mesh with wide openings would be less likely to do that but would still keep fish safe.